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1. Field of the Invention
This invention pertains to automobile traffic intersection controls. More particularly, it pertains to the monitoring of the operation of the equipment controlling the traffic signals at an intersection.
The automobile traffic signals and the pedestrian "walk" signals at an intersection typically are controlled by an "intersection controller" located at the intersection. The controller operates the automobile traffic signals and the pedestrian signals at the intersection in a manner that produces non-interfering traffic movement. A standard nomenclature for such controllers in the United States has been set forth in Section 14 of the National Electrical Manufacturers Assocation (NEMA) Standards Publication No. TS1-1983, and interface standards for such controllers are set forth in Section 13 of the same publication.
Each mode of traffic flow at the intersection is denoted as a "phase". Included within each phase is the operation of the green signals indicating the directions in which traffic is allowed to move in the particular phase, the yellow signals warning of the pending termination of traffic movement, and, in some cases, the initial operation of red signals for a period of time sufficient to allow for clearance of traffic through the intersection before the green signals in the next phase are operated to initiate the next phase of traffic flow. If neither the green nor the yellow signal in a red, green, yellow cluster is "on", then the red signal is "on" to the indicate the directions in which traffic may not move.
As indicated in Section 14 of the NEMA publication, the sophistication of the operation of the controller may range from the simple to the complex. For instance a "single" ring controller operates the signals at an intersection so as to produce traffic flow in a sequence of phases arranged so as to occur in an established order. In response to demands by automobile and pedestrian detectors the controller may extend or shorten the period of time during which traffic flows in each phase. However, in a single ring system, the sequence of phases cannot be altered. For example, in an intersection without left turn signals, a single ring controller can be programmed to allow traffic flow in the north/south directions to continue without interruption until an automobile is detected on the east/west street, at which time the phase allowing north/south traffic flow is terminated and the phase allowing east/west traffic flow is initiated. Depending upon the programming in the controller, the east/west phase may then remain in operation until traffic is detected on the north/south approaches to the intersection or until the expiration of a predetermined period. Sophisticated single ring controllers may extend the interval in which one phase operates in response to continued demand, or may shorten the interval in response to demands for operation of other phases. Although the duration of each phrase may be altered by the single ring controller, the sequence of phases cannot be changed.
A "dual-ring" controller allows two independent sequences, or rings, of phases to operate simultaneously at an intersection. A dual-ring controller allows some variation in the sequence of patterns or phases of traffic flow. Certain limitations must be imposed, however, to avoid interfering traffic movement.
FIG. 1A depicts a typical intersection of two streets, one oriented in a north/south direction, the other oriented in the east/west direction. As an example, the phases of a two ring controller may be allocated to the traffic flow for the intersection as follows. Phase 1 allows northbound traffic to turn left at the intersection. Phase 2 allows southbound traffic to proceed straight through the intersection while at the same time allowing north/south pedestrian traffic to cross the street on the west side of the intersection. Phase 5 allows the southbound traffic to turn left at the intersection and phase 6 allows northbound traffic to proceed straight through the intersection while also allowing north/south pedestrian traffic to cross on the east side of the intersection. Phases 3 and 4 and 7 and 8 similarly relate to east-west traffic. As a consequence, regardless of whether phase 1 or phase 2 is in operation, either phase 5 or phase 6 also may be in operation. That is, during the period when phase 1 allows northbound traffic to turn left, either phase 5, which allows southbound traffic to turn left, or phase 6, allows northbound traffic to proceed straight through the intersection, also may be in operation. However, while phase 1, which allows northbound traffic to turn left, is in operation, southbound traffic cannot be allowed to proceed straight through the intersection, i.e., phase 2 cannot be allowed. Although phase 5 may proceed to phase 6 independent of the progression from phase 1 to phase 2, phase 6 cannot proceed to phase 7 until the sequence of phase 1 and 2 is ready to proceed to phase 3, i.e., the east and west bound traffic flow represented by phases 3 and 4 and 7 and 8 cannot begin until the north and south flows represented by phases 1, 2, 5 and 6 are terminated. Similarly, phases 1, 2, 5 and 6 cannot be initiated until phases 3, 4, 7 and 8 terminate.
The more sophisticated controllers which adjust the sequence and the duration of the various phases of traffic flow in response to traffic and pedestrian demands decrease the waiting times for the vehicular traffic and for the pedestrians when these controllers and the related equipment are operating correctly. Such adaptation to demand, however, can result in inefficient operation and increased traffic delays if any of the traffic or pedestrian detectors malfunction or if the controller itself malfunctions.
In addition to the intersection controller and the pedestrian and automobile detectors, each intersection usually includes a conflict monitor and flasher equipment. The conflict monitor is a fail safe device which monitors the operation of the various signals. If for any reason, a combination of traffic signals are "on" simultaneously which would allow interfering traffic flow, the conflict monitor directs the flasher equipment to disconnect all of the traffic signals from the intersection controller and to connect the red signals to a flasher, thus placing all of the red signals in a "red" flashing mode. The flasher equipment also can be set to flash yellow on the "main" street and red on the cross street.
2. Description of the Prior Art
In the past, citizen complaints and reports from municipal employees or police have been used to detect and report obvious malfunctions of the system. For instance when a controller is "stuck" in a particular phase such that no cross traffic is allowed, when all the signals are out, or when the intersection is in the red flashing mode, such malfunctions are readily apparent and, because of the extent of the disruption, are quickly detected and reported. As a consequence, such major malfunctions are repaired shortly after they occur. More subtle malfunctions, such as the failure of an automobile detector which prevents the controller from adjusting the timing of each phase in response to traffic demands, are not easily detected by the casual observer. Because it is often not economically practical to have the traffic signal system inspected in a timely manner by personnel who have been trained to detect such non-catastrophic malfunctions, such malfunctions often exist undetected and unreported for long periods of time. As a consequence, after a significant period of operation, the more subtle malfunctions tend to become wide spread in such sophisticated traffic control systems. For instance, in an article titled "Maintenance of Traffic Signals in London" by K. H. S. Oastler, in the March, 1985 issue of Traffic Engineering and Control inspections were reported to have found anywhere from 40 to 98% of the intersections to have major or minor faults. Although such faults may not completely disrupt traffic flow, they nevertheless cause traffic to flow less efficiently. Accordingly, it is important that such sophisticated systems provide a means to monitor and report non-catastrophic as well as catastrophic malfunctions.
In traffic systems that are controlled from a central location some prior art systems have including means for malfunction detection. In such a central control system the outputs from the automobile and pedestrian detectors are all brought to the central location and the phases at each intersection are all directly controlled from the central point. As a consequence it is relatively simple to include malfunction detection equipment at the central location. The expensive communication capacity, however, that is required between the central location and the various intersections often makes such central control uneconomic. Because of this economic limitation, the vast majority of intersection controllers today are each located at an associated intersection and operate independently from each other. Thus, there is a need for a system for monitoring and reporting malfunctions in the operation of each intersection controller, but in a manner which does not carry with it a requirement for extensive dedicated communication facilities between the central monitoring point and the controllers at the remote intersections.